The foam-laid process for forming non-woven fibrous webs is basically disclosed in U.S. Pat. Nos. 3,716,449, 3,871,952, and 3,938,782 (the disclosures of which are incorporated by reference herein). The foam-laid process has a number of advantages over the water-laid process that is most conventionally used for making synthetic or cellulose fiber webs. The invention relates to a method and assembly for implementing the foam-laid process so as to improve aspects thereof.
According to the invention it has been found desirable for a number of different purposes to strategically introduce a substantially pure foam (that is water, air, and surfactant, being substantially fiber-free) adjacent or into a headbox for forming a non-woven web. By introducing the pure foam flow into the flow of a foam-fiber mixture near (e.g. just prior to) introduction of the foam/fiber mixture into the headbox (up to the actual introduction of the foam-fiber mixture thereinto) it is possible to increase the uniformity of the basis weight profile of the non-woven web produced. In fact it is possible to provide a web basis weight variation of less than 0.5%, e.g. as low as 0.2% or perhaps even less depending upon the fibers.
Alternatively, or in addition, by introducing the pure foam flow into the headbox adjacent a surface (such as the roof surface of an inclined headbox) thereof, it is possible to minimize shear of fibers in the headbox so that the fibers do not become unidirectional, in the direction of movement of the foraminous element (wire), and so that the surface is kept clean. These advantageous results may be achieved in a simple and very inexpensive manner, one that essentially introduces almost no additional operating costs and very few additional capital costs.
According to one aspect of the present invention a headbox assembly for producing a non-woven web of fibrous material is provided comprising the following components: A moving foraminous element on which a non-woven web may be formed. A headbox comprising a first surface and a second surface, the second surface remote from the foraminous element, and the headbox adjacent the foraminous element so that a foam fiber mixture in the headbox deposits fibers on the foraminous element. Means for introducing a foam fiber mixture into the headbox. Means for withdrawing foam through the foraminous element to form a non-woven fibrous web on the foraminous element. And, means for passing a substantially fiber free foam into contact with the second surface at a position remote from the foraminous element.
The means for introducing the foam-fiber mixture into the headbox may include a plurality of openings in the first surface, as well as other components that are conventional for introducing a fluid flow into a volume, including conduits, nozzles, orifices, headers, manifolds, or other conventional devices. The means for withdrawing foam through the foraminous element may comprise any conventional structure, such as suction boxes or tables, suction rollers, pressing rollers, or any other conventional components that are capable of performing that function.
The means for passing a substantially fiber-free foam into contact with a second surface may also comprise any type of conventional fluidic element that can accomplish that purpose including conduits of various shapes, sizes, and orientations, nozzles, orifices, headers, manifolds, or any like conventional devices.
The assembly may also comprise means for introducing substantially fiber free foam into the means for introducing a foam fiber mixture into the headbox just prior to the headbox so as to provide a more uniform basis weight profile of the non-woven web produced. Such means may also comprise any conventional fluidic components such as conduits, conduit branches, orifices, manifolds, etc., such as one set of conduits making an angle (e.g. between about 30-90.degree.) to the fiber-foam mixture containing conduit immediately adjacent (up to the actual point of introduction of the foam-fiber mixture) the headbox.
The means for passing a substantially fiber free foam into contact with the second surface at a position remote from the foraminous element may comprise at least one conduit opening adjacent the second surface for causing foam to flow along the second surface toward the foraminous element so as to minimize shear of fibers in the headbox so that the fibers do not become unidirectional, in the direction of movement of the foraminous element, and so as to keep the second surface clean. The assembly may further comprise a baffle adjacent the means for passing a substantially fiber free foam into contact with the second surface at a position remote from the foraminous element to ensure initial flow of the introduced foam along the second surface. The second surface may be a roof surface of the headbox, and the foraminous element may move at an angle to both the horizontal and vertical, the headbox being an inclined headbox.
According to another aspect of the present invention a method of producing a non-woven web of fibrous material, using a headbox, a moving foraminous element, and a surface of the headbox, is provided. The method comprises the following steps: (a) Feeding a first foam slurry of air, water, fibers, and surfactant into the headbox and into contact with the moving foraminous element. (b) Passing a lubricant (preferably a first substantially fiber-free foam) into contact with the surface of the headbox at a point remote from the foraminous element. And, (c) withdrawing foam through the foraminous element to form a non-woven fibrous web on the foraminous element.
Step (b) may be practiced to cause the first foam to flow along the surface toward the moving foraminous element so as to minimize shear of fibers in the headbox so that the fibers do not become unidirectional, in the direction of movement of the foraminous element. The surface of the headbox may comprise a roof surface thereof, and step (b) may be practiced to cause foam to flow along the surface toward the foraminous element so as to also keep the surface clean. The amount of foam added in (b) may be 1-10% by volume of the flow in (a). There may also be the further step of passing a second substantially fiber-free foam into the first foam slurry just before the first foam slurry is fed into the headbox so as to provide a more uniform basis weight profile of the non-woven web produced.
Step (a) is typically practiced so that the first fiber-foam slurry flows in substantially the same direction as the first substantially fiber-free foam. Step (b) may also be practiced by providing a baffle in the headbox which assists in directing the first substantially fiber-free foam along the surface, and so that it does not initially mix with the first fiber-foam slurry introduced into the headbox.
According to another aspect of the present invention a headbox assembly is provided comprising the following components: A headbox associated with a moving foraminous element. Means for feeding a first foam slurry of air, water, fibers, and surfactant into the headbox and ultimately into contact with the moving foraminous element. Means for withdrawing foam through the foraminous element to form a non-woven web on the foraminous element. And, means for passing a second, substantially fiber-free foam, into the first foam slurry near (e.g. just before) where the first foam slurry is fed into the headbox. The means for feeding, withdrawing, and passing may have the modifications such as discussed above.
The means for feeding may comprise a plurality of foam forming nozzles and a plurality of first conduits connecting the nozzles to the headbox; and the means for passing a second, substantially fiber-free foam, into the first foam slurry just before the first foam slurry is fed into the headbox may comprise a plurality of second conduits associated with at least some of the first conduits and making an angle with respect thereto. The angle between the first and second conduits may be between about 30-90.degree., and in a vertical plane.
According to yet another aspect of the present invention a method of producing a non-woven web of fibrous material, using a headbox, and a moving foraminous element, is provided. The method comprises the steps of: (a) Feeding a first foam slurry of air, water, fibers, and surfactant into the headbox and into contact with the moving foraminous element. (b) Withdrawing foam through the foraminous element to form a non-woven fibrous web on the foraminous element. And, (c) passing a second, substantially fiber-free foam, into the headbox (e.g. into first foam slurry near (e.g. just before) where the first foam slurry is fed into the headbox), to provide a more uniform basis weight profile of the nonwoven web produced.
Step (a) is typically practiced by moving the fiber-foam slurry in a generally horizontal direction, although in some circumstances it may be moved vertically or at angles. Steps (a) through (c) are typically practiced to produce a non-woven web having a consistency before drying of about 40-60%, and a basis weight variation of less than 1/2% (e.g. about 0.2%, or even less). The amount of flow in (c) may be between about 2-20%, by volume, the flow in (a).
It is the primary object of the present invention to enhance the foam-laid process for the production of non-woven webs of fibrous material. This and other objects of the invention will become clear from an inspection of the detailed description of the invention and from the appended claims.